THD claims of audio signal generators

Very true.

If you knew the settings for the cutter lathe you could do the same for vinyl too, despite some audiophools not understanding this point ( they think it has unlimited dynamic range it seems ! ).

Graham

No !

It's ridiculously simple in fact.

Graham

too,

unlimited

But it is different for every disc - the point with CD is that you can set it once and forget it.

d

Pearce Consulting

Wrong. It's easy if you know how.

Not necessarilily. What's this 'power guard' anyway ? Some audiophoolery I expect. Pro-audio amps have had signal limiters to avoid clipping for *decades*. Including the cutting lathe amplifiers that made the vinyl !

That helps for sure. :-)

That's a pure lie. Tube amplifiers are staggeringly inefficient.

tubes",

How do you know ? Personal bias ?

Graham

too,

unlimited

CD makes it simpler for sure since there is a clearly defined 'brick wall'.

Just pointing out that vinyl doesn't have some supposed infinite headroom.

Graham

Since the cartridge you use has it's trackability limits, and the cutter lathes also have their limits, there is still a finite limit to what you will ever get from a vinyl disk. This can be ascertained. OTOH, CD's have differing average levels, so you cannot "set and forget", only ascertain the peak level setting for clipping for Dfs. Admittedly the peak levels on all modern CD's come close to this, whereas the peak levels for individual vinyl disks cover a wide range. Knowing the maximum gain setting in both cases allows you to set a volume control level below the maximum required for amplifier induced clipping however.

MrT.

Audio Precision test oscillators offer THD residuals in the 0.0006% region (

-104dB )

Not a toob in sight !

Graham

I may shortly be diving inside our AP test set ( backlight needs replacing ) . Dunno what they use actually. Maybe some exotic PMI or AD parts in selected places ?

Graham

Spot on !

Not sure what you're saying. It's certainly easy to set up a listening system that simply won't clip from a CD source. The *CD* may be clipped of course but that's an entirely different matter.

Correct.

Graham

Nonsense. People do it all the time.

A totally ludicrous goal. In the real world enough can be known about the initial requirements that clipping can generally be avoided.

Compression is not a solution because it has its own set of audible consequences.

By the ca. 1960 standards of tube bigots, really really big amplifers are now readily available.

Total nonsense.

There's no way that a 20 watt tube amp can sound better than a 500 watt amp, if the situation requires much more than 20 watts. While tube enthusiasts like to posture about how their hobby-horse amps sound as good if not better than far larger SS amps, back in the real world an hi fi amp that is clipping sounds bad no matter what its active devices are.

This is just more senseless posturing. Of course a sweeping generality can be false under some situations. So what?

There are enough people listening to amplifiers that the "many people (who) still prefer to use vacuum tubes" is a miniscule and shrinking minority.

I thought this was about technology, not recreation.

Setting levels for vinyl playback or transcription is similarly pretty easy.

Get a trackbility test LP such as the one from Hi Fi News and set levels so that you can play the highest trackable segment while leaving a few dB for headroom throrgh the electronics. It's really a similar thing to the procedure that was suggested for a system with a CD player.

An electronic circuitry that backs off the amplifier's gain when the input signal would tend to push it into clipping.

"Power Guard® clipping protection. Power Guard ensures that the amplifier will always deliver full power without causing clipping distortion. If an amplifier channel is overdriven, Power Guard automatically reduces the input volume just enough to keep distortion below 2% and prevent any clipping distortion. Thanks to an optical resistor, Power Guard acts literally at the speed of light, producing absolutely no audible side effects. An amplifier with Power Guard will actually deliver clipping-free output well above its rated power."

Vinyl bigots would die if they knew about all the work-arounds that were routinely used in its production.

Bret is also implicitly claiming that a 20 watt tubed amp can sound as good as a 500 watt amp when 500 watts would be required to avoid clipping. This is complete and total nonsense.

It's what the voices in his head tell him. I think he should unwrap the aluminum foil from around his cranium.

Various construction projects like this have been published in the past, including one by me. There was also a far more elaborate project by Cordell that was published in Audio Magazine.

If one works with building audio generators of the traditional analog kind, it turns out that the nonlinearity of the means used to stabilize the levels is the major source of distortion, not the amplifier portion of the oscillator. This is true whether a light bulb, a CdS opto-isolator, a thermistor, a FET or a VCA is used. Been there and done that for all of them.

Maybe even discrete op amps, depending on the age.

One relevant parameter is the maximum amplitude that is provided. One classic benchmark maximum output in the 10 vrms or +22 range. To provide this at the generator's output terminals @600 ohms, you have to have a few dB more at the op amp's output terminals.

You can't really do this with +/- 15 or +/- 18 supplies. One can stretch NE5532s to +/- 22 but they tend to degrade over years.

The only high-voltage op amp chip that I know (one that shows signs of hanging in with +/- 22) of is the OPA 604/2604. According to Doug Self, they vastly underperform NE 5532s for nonlinear distortion.

Modern DAC chips are so good, and digital computation and function management is so cheap and pervasive, that a modern sound card in a PC is the most practical way to generate well-controlled sine waves these days.

It takes a lot of work to outperform a M-Audio Audiophile 24192 driven by simple freeware software like Audacity and/or Audio Rightmark.

I looped-back a little XP PC with an Audiophile 24192 in it the other day, running the freeware Audio Rightmark Program. *All* spurious responses were

112 dB or better down (most in the -120 dB range), and THD+N was something like -106 dB.

AFAIK toobed audio generators and analyzers never got within 2-3 orders of magnitude of -106 dB residuals. Something like 0.05% midband, and 0.1% at 20 and 20 KHz was about as far as toobs got.

When the first generation SS audio test equipment like the HP 331-334 first hit the market, residuals *instantly* improved by like an order of magnitude.

Heath made a cheap clone of this box (IM5258) that I was able to improve so that it had mid-band residuals in the 0.001-0.003% range.

forget",

Simply that the apparent SPL level will be different using different CD's in a "set and forget" position regardless of Dfs. I think it's simply bad terminology saying "set and forget". It is actually ascertaing the *maximum* gain setting, and one *often* varies the system gain below that level.

Which I agreed with, vinyl too. Also FM etc.

Nearly all are these days.

Quite so.

MrT.

The biggest problem with low residuals back then was not, in my experience, wether they used tubes or solid state, rather on how well the unit was stabilized. There was a Krohn-Hite tubed oscillator, as I recall, that was easily capable of well below 0.003% , you just had to let it sit there and stabilize. AT the same time, some of the GR solid state oscillators, like the 1309, could be tuned to meet those kinds of levels, and MIGHT have been capable of far better, but the amplitude stabilization network just just too noisy: you'd watch the residual bouncing around and every once in a while you'd see it drop a good

20 dB below its average for a brief period (about a second). Bang the case, upset the filament in the bulb they used for stabilization, and you'd see the residual go all over the place.

The real secret to low-residual oscillators came with much better , lower noise and faster responding loop stabilization. The original ST1700 had an oscillator circuit not substantially different than whatever else was out there but had superior stabilization and that was the secret to their significantly lower residual.

Neat !

Do you fancy posting any more info about that ?

Graham

Once upon a time I wrote an outline of an article about it for Ed Dell, but lost interest in his ragazine due to his negative stance on ABX before getting much furhter with it.

I still have the modded 5258 packed away someplace. I fired it up about a year ago and it still worked. It was a real POS compared to what I now do with PCs. Hard to operate, limited reporting, relatively high residuals.

The mod development work revealed some interesting stuff.

One of the more significant zero-cost enhanments involved taking a grounding problem out of the power supply circuit card that flooded the whole box with low-level ripple. Bad land pattern design around the power supply caps.

Part of the mod involved replacing one or two discrete transistor stages with 5534s.

I had to up the gain of the measurement circuits by about 20 dB to make the lower residuals useful. There was a 10/20 dB gain boost switch that the mod added.

I had to work over some time constants in the nulling circuit, and make them change for the lower frequency ranges. That was controlled by a second added front-panel switch.

The box was noisy because it had immense bandwidth - something like 4-5 MHz at -3 dB. To get the lowest residuals a switchable low-pass filter was added to the metering circuit.

too,

unlimited

Yes, but there's a massive variation in maximum groove velocity, from say EMI 'Music for Pleasure' at one end, to the Sheffield Track and Drum records at the other. Setting your system gain to avoid clipping at 30cm/sec will make for very quiet listening to most commercial output!

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Stewart Pinkerton | Music is Art - Audio is Engineering